1. Gap Transient Suppression using Increased Bunch Density
Andrew Hutton

2. HL-LHC Collaboration Meeting Oct 2018
Gap Transient Problem
In a high-current multi-bunch storage ring, an abort gap is required to protect the hardware from errant beams
A gap is also usually required to mitigate fast ion or electron cloud instabilities
The gap produces a large transient in the RF system
The transient is hard to correct because the beam loading from the beam is large compared to the available RF drive power
The RF transient produces an energy slope along the bunch train
This can shift the collision point and/or the time of collision at the IP
From my presentation
HL-LHC Collaboration Meeting Oct 2018

3. HL-LHC Collaboration Meeting Oct 2018
Proposed solutions
Brute force – add more RF generator power
Prohibitively expensive
Pulse the RF systems to reduce the RF power requirements
Reduces the klystron power by locking RF phase to actual bunch phase
Does not solve the problem of bunch energy variation along the bunch train
Solution proposed by Daniel Boussard for LHC, implemented by Themis Mastorides
Match the energy slope for the two beams
All bunches collide at the same position where the beta functions are optimized
Solution adopted by PEP II and LHC
Hard to match electrons and ions
Increase bunch charge before and after the gap so average current is constant
Solution proposed by John Bird for ALS
Tested successfully by Bob Rimmer and Dmitry Teytelman at BEPC
Increasing the bunch charge may cause instabilities
Doesn’t work with gear-changing
Can insert additional bunches in empty buckets in ion ring – how?
This solution removes all energy and position transients
From my presentation
HL-LHC Collaboration Meeting Oct 2018

4. Bunch and Charge Distribution
There are two ways of increasing the bunch current either side of the gap:
Increase the bunch charge with the same bunch distribution
Increase the bunch density with the same bunch charge
Increasing the bunch charge brings problems of instability, halo, etc. which might be acceptable
Increasing the bunch density does not have these problems but creating the correct bunch pattern stalled this approach
This option is available because the RF frequency will be 952 MHz while the bunch repetition frequency is 476 MHz
From an to Bob Rimmer :
Your idea to increase the bunch charge on either side of the gap fixes the RF gap transient  
A conceptually simple solution would be to fill the intervening buckets in these areas; this doubles the current but preserves the beam-beam interactions  
However, I have no idea how to create this filling pattern
HL-LHC Collaboration Meeting Oct 2018

5. HL-LHC Collaboration Meeting Oct 2018
Large Booster
The change to the JLEIC ion injection chain to include a full-size booster changed the situation
This full-size booster has several advantages, the biggest impact is on the average luminosity if the bunches are prepared in the booster during collisions so they are ready when the colliding bunches need to be replaced 
This moves all of the bunch splitting into the full-size booster and out of the collider ring  
There is an additional advantage
If all the bunch splitting is done in the full-sized booster, additional bunches could be created in a second fill and transferred (bucket to bucket) into the empty buckets
There are two options:
Interleave the bunches in the second fill between the circulating bunches
Create the double-frequency bunch pattern in the full-size booster and inject the bunch train into an empty gap
HL-LHC Collaboration Meeting Oct 2018

6. HL-LHC Collaboration Meeting Oct 2018
Options
Interleaved transfer
Circulating Bunches
Abort Gap
Circulating Bunches
Second Injection
Second Injection
Bunch train transfer
Kicker Gaps
HL-LHC Collaboration Meeting Oct 2018

7. Comments on Bunch Train Transfer
Bunch train transfer is the easiest
Requires additional gaps for kicker rise and fall times which also need to be compensated
From Jiquan’ presentation
Abort gap = 267 ns
Kicker gaps = 2 x 20 ns
Double current regions 2 x 134 ns
Values only approximate – need to be an integral number of wavelengths
Additional hardware required:
Small 976 MHz RF system in full-size booster for additional bunch splitting
Fast kicker:
Either two kickers or one kicker capable of two pulses with short inter-pulse charging time
HL-LHC Collaboration Meeting Oct 2018

8. Comments on Interleaved Transfer
Interleaved transfer is much harder
Requires an injection system of increased complexity
There is a theorem that there exists no system of linear or nonlinear optics which can simultaneously close multiple local orbit bumps and dispersion through a single beam transport region 
This means that either an unclosed injection kick is acceptable or a time- varying kick (e.g. an RF deflector) would be needed 
HL-LHC Collaboration Meeting Oct 2018

9. Unclosed Injection Kick
The following slides are from a talk by Chiara Bracco, CERN
216/ /CAS_ERICE_Hadrons_Injection.pdf
HL-LHC Collaboration Meeting Oct 2018

10. Unclosed Injected Kick = Filamentation
HL-LHC Collaboration Meeting Oct 2018

11. Injection with Dispersion
If the incoming particles have a different energy from the circulating bunches and there is dispersion at the septum:
It is possible to inject such that the bunch separation equals the energy difference x dispersion
There is then no transverse filamentation
BUT, since the energies are different, the incoming bunches will oscillate with the synchrotron frequency
Result – filamentation in the longitudinal direction leading to long bunches
Energy-related halo is liable to be a problem
HL-LHC Collaboration Meeting Oct 2018

12. Matched Interleaved Injection
Involves an RF transverse kicker 90 degrees downstream from the septum operating at 476 MHz with a dipole associated such that the circulating beam sees zero kick
The incoming bunches are offset by one half wavelength at 476 MHz and therefore see a kick from the RF kicker and the dipole
This can be used to put the injected bunches on the circulating orbit so that they can be captured by the 952MHz RF system in the ion ring.     The rise and fall time of the RF kicker/dipole pair should be of the order of 50% of the revolution time in the ion ring (a few microseconds)
The peak power required is huge (Jiquan estimated 12.5 MW for 10 meters of RF structure), but the duration of the power requirement is really short
A possible solution is based on the KEKB ARES cavities, which have a large storage cavity, an intermediate coupling cavity and the accelerating cavity itself ( and switching the coupling cavity with ferroelectrics  
RF curvature is also a problem
HL-LHC Collaboration Meeting Oct 2018

13. HL-LHC Collaboration Meeting Oct 2018
Overall comments
Unmatched injection is unacceptable
Unmatched injection leads to longitudinal halo
Interleaved injection with an RF kicker is technically possible, but looks expensive and would be a long R&D project to demonstrate
Best for the moment is the batch train transfer
Would be nice to find a way to avoid the additional kicker gaps
HL-LHC Collaboration Meeting Oct 2018